1. Field of the Invention
The present invention relates to an EL display that is formed by fabricating an EL (Electro Luminescence) element on a substrate. Particularly, the present invention relates to an active matrix type EL display that uses a semiconductor element (an element employing a semiconductor thin film), and furthermore to a semiconductor display device employing the EL display.
2. Description of the Related Art
In recent years, technology for forming a TFT on a substrate has been largely improved, and an application development of the TFT to an active matrix type semiconductor display device has been carried out. In particular, the TFT using a polysilicon film has a higher electric field effect mobility than the TFT using a conventional amorphous silicon film, and therefore, the TFT may be operated at a high speed. Thus, the pixel control which has been conducted at a driver circuit outside of the substrate may be conducted at the driver circuit which is formed on the same substrate as the pixel.
Such an active matrix type semiconductor display device can, by preparing various circuits and elements on the same substrate, obtain various advantages such as a decrease in manufacturing cost, a decrease in the size of the semiconductor display device, an increase in yield, and a decrease in throughput.
Further, research on the active matrix type EL display having an EL element as a self-light-emitting element is becoming more and more active. The EL display is referred to as a light-emitting display, an organic EL display (OELD) or an organic light-emitting diode (OLED).
The EL display is a self-light-emitting type unlike a liquid crystal display device. The EL element is constituted such that a layer containing an organic compound (hereinafter, referred to as an EL layer) is sandwiched between a pair of electrodes (anode and cathode). However, the EL layer normally has a lamination structure. Typically, the lamination structure of a xe2x80x9chole transport layer/a light emitting layer/an electron transport layerxe2x80x9d proposed by Tang et al. of the Eastman Kodak Company can be cited. This structure has a very high light-emitting efficiency, and this structure is adopted in almost all the EL displays which are currently subjected to research and development.
When the EL element obtains Luminescence (Electro Luminescence) which is generated by applying a voltage to the EL element, it is composed of an anode layer, an EL layer, and a cathode layer. There are two types of luminescence in an organic compound, one being a luminescence that is generated when the organic compound returns from a singlet excitation state to a ground state (fluorescence) and the other being a luminescence that is generated when the organic compound returns from a triplet excitation state to a ground state (phosphorescence). Either type of luminescence may be used in the EL display of the present invention.
In addition, the structure may be such that on the electrodes, a hole injection layer/a hole transport layer/a light emitting layer/an electron transport layer, or a hole injection layer/a hole transport layer/a light emitting layer/an electron transport layer/an electron injection layer may be laminated in order. Phosphorescent dye or the like may be doped into the light emitting layer.
In this specification, all the layer s provided between a pair of electrodes are generally referred to as EL layers. Consequently, the hole injection layer, the hole transport layer, the light emitting layer, the electron transport layer, the electron injection layer and the like are all included in the EL layers.
In this specification, a light emitting element, which is composed of an anode, an EL layer and a cathode, is referred to as an EL element.
The deterioration of the EL material of the EL layer has become a problem in the realization of the EL display, which leads to the reduction in the luminance of the EL element.
The EL material of the EL layer is inferior to moisture, oxygen, light, and heat, which are the factors that promote the deterioration of the EL layer. To be more specific, the rate at which the EL layer deteriorates is influenced by the structure of a device driving the EL display, characteristics of the EL material structuring the EL layer, materials of an electrode, conditions of the manufacturing processes, a driving method of the EL display and the like.
The EL layer deteriorates even if a constant voltage from a pair of electrodes is applied thereto, whereby the luminance of the EL element is reduced. Thus, an image displayed on the EL display is not clear because of the reduction in the luminance of the EL element.
Further, Color display systems of the EL display are roughly divided into four: a system where three kinds of EL elements corresponding to R (red), G (green), and B (blue), respectively, are formed; a system where EL elements emitting white light are combined with a color filter; a system where EL elements emitting blue or blue-green light are combined with a fluophor (fluorescent color conversion layer: CCM); and a system where EL elements corresponding to R, G, and B are superimposed on a transparent electrode used as a cathode (an opposing electrode) (RGB stacking method).
The EL material that structures the EL layer differs depending on the luminescing color of the EL layer. Therefore, in the color display system that employs three kinds of El elements corresponding to the colors R (red), G (green), and B (blue), the three kinds of EL elements of the EL layer corresponding to RGB each may deteriorate at different rates. In this case, the luminance of the EL elements that correspond to RGB becomes dissimilar, respectively, as time passes. Consequently, an image having a desirable color cannot be displayed on the EL display.
The present invention has been made in view of the above, and therefore has an object to provide an EL display capable of performing a clear and desirable color display by suppressing a reduction in luminance of an EL element even if an EL layer is deteriorated.
The EL display of the present invention has a sensor portion for detecting a luminance of a portion for displaying an image of the EL display (display portion) and revising the luminance to a desirable value. The sensor portion includes one or a plurality of pixels. It is to be noted that the pixel(s) of the sensor portion will hereinafter be referred to as sensor pixel(s) throughout this specification.
The sensor pixel(s) is composed of an EL element and a light receiving diode that detects the amount of change in the luminance of the EL element. It is to be noted that throughout this specification, the EL element of the sensor pixel(s) will hereinafter be referred to as a sensor EL element.
The sensor EL element has the same structure as that of the EL element (hereinafter referred to as display EL element) of the pixel (hereinafter referred to as display pixel) of the display portion. At least a material that constructs a pair of electrodes and a material that constructs a lamination structure of an EL layer and the EL layer are the same, respectively.
Then, a signal, which is the same as a signal inputted to an arbitrarily selected display EL element, is fed to the sensor EL element. In this specification, the input of a signal to the EL element (display EL element and sensor EL element) means that an electric potential of the signal is applied to one of the electrodes of the EL element, and an EL drive voltage is applied to the EL layer. Here, the EL drive voltage is the electric potential difference between the electric potential of the signal applied to one of the electrodes of the EL element and the constant electric potential applied to the other electrode thereof.
Thus, an equivalent voltage is applied to the EL layers of the sensor EL element and the arbitrarily selected display EL element, whereby the deterioration rates of the EL layers are nearly equivalent. Therefore, the luminance of the sensor EL element and the luminance of the display EL element maintain almost equivalent states even as time elapses.
Light emitted by the sensor EL element, on one hand, is irradiated to the light receiving diode of the senor pixel. Then, the light receiving diode detects the luminance of the sensor EL element. On the basis of the information of the luminance of the sensor EL element that was detected, the luminance of the display EL element is revised, and the luminance of the sensor EL element is also revised at the same time.
By adopting the above structure, the present invention has made it possible for the EL display to perform a clear and desirable color display by suppressing the reduction in luminance of the EL element even if the EL layer is deteriorated.
The EL display of the present invention may be of a color display system that employs a display EL element emitting white light, or a color display system that employs display EL elements corresponding to the colors RGB, respectively. In case of the color display system that employs the display EL elements corresponding to each of the colors RGB, it is preferable that the senor pixels corresponding to each of the colors RGB are provided in the sensor portion. However, the present invention is not limited to the structure thereof. It may be a structure in which the sensor pixels, which correspond to either 1 or 2 colors of the RGB colors, are provided in the sensor portion. In particular, it is effective to provide the sensor pixel that corresponds to the color of which the deterioration of the EL layer is remarkable in the sensor portion to thereby display an image having a desirable color.
It is further preferable that the display EL element and the sensor EL element are formed at the same time under the same conditions. The deterioration rates of the EL layers of the display EL element and the sensor EL element can be made equivalent by adopting the above structure. Therefore, the luminance of the sensor EL element that will be detected by the light receiving diode becomes equivalent with the luminance of the display EL element, to thereby detect the change in the luminance of the display EL element more accurately. Thus, it becomes possible to revise the luminance of the display EL element to a desirable value.
Furthermore, when the sensor portion is formed simultaneously with the display portion on the substrate, as the manufacturing process of the El display, only the process of forming the light receiving diode has to be added to the manufacturing process in the case where the sensor portion is not provided. Therefore, there is no need to remarkably increase the number of manufacturing processes, thereby making it possible to suppress the number of manufacturing processes.
It is to be noted that a portion of the display portion may be used as the sensor portion. That is, among the pixels of the display portion, one or a plurality of pixels that are arbitrarily selected may be employed as sensor pixels and the rest of the pixels may be employed as display pixels. In this case, the size of the EL display can be suppressed because the space for the provision of the sensor portion can be omitted compared with the case of not including the sensor portion in the display portion.